Something undeniable is clearly true: it's undeniable that water is wet and the sky is blue.

Undeniable things are obviously, clearly, totally, blatantly true, so true that it would be crazy to deny them. If you spilled soup in your lap, it's undeniable that you're wet and probably embarrassed. It's undeniable that a lot of dogs like chasing squirrels. On the other hand, it's undeniable that not everything is that simple. One person might deny what another thinks is flat-out undeniable.

Definitions of undeniable

adj not possible to deny

incontestable, indisputable, undisputable
not open to question; obviously true
incontrovertible, irrefutable, positive
impossible to deny or disprove
incapable of being questioned
capable of being denied or contradicted
capable of being disavowed
confutable, confutative, questionable, refutable
able to be refuted
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